var nav = document.querySelectorAll(".nav > li");
var main = document.querySelectorAll(".main > div");

for (var i = 0;i < nav.length;i++){
    nav[i].index = i;
    nav[i].addEventListener('click',function(){
        for (var k = 0;k < nav.length;k++){
            nav[k].classList.remove('active');
            main[k].classList.remove('active');
        }
        this.classList.add('active');
        console.log(this.index);
        main[this.index].classList.add('active');
    })
}